It is known that it is possible to exchange data simultaneously between a plurality of ground stations through a relay station, for example, the transponder of a communications satellite, where the transponder amplifies the sum signal from all presently transmitting ground stations received by its antenna in a limited frequency band and retransmits them in another frequency band. Such a system is described for example, in the article by Schwartz, Aein, Kaiser, "Modulation Techniques for Multiple Access to a Hard-Limiting Satellite Repeater", Proc. IEEE, Vol. 54, No. 5, May 1966, pages 763-777. The time multiplex process (TDMA) can be used for such multiple transmission in that each ground station has associated to it a pulse bundle, a so-called burst, within a periodically repeated pulse frame. The times at which these bursts are transmitted are controlled in the respective ground stations so that the bursts arrive at the satellite, taking into consideration of their travel times to the satellite, in a prescribed sequence one after the other behind a reference burst, the so-called code sequence SRUW (synchronization reference unique word), which is transmitted by a reference station. For a control of the position of the transmitted bursts relative to the beginning of the frame established by the code sequence SRUW, it is a prerequisite that all transmitting stations can receive the code sequence SRUW as well as their own transmitted burst from the satellite without interference. If the reception of the code sequence is prevented by malfunctions of instruments or as a result of bad weather conditions, the transmission of the code sequence SRUW must be taken over by another ground station which thus becomes the reference station.
In known communications systems of this type three of the intended ground stations, for example, which are disposed at different locations within the radiation range of the satellite antenna are equipped to operate as reference stations. However, only one station can be effective as the reference station at any one time to determine the synchronism of the entire system. A change between these stations is initiated as soon as the code sequence SRUW cannot be received by one or more of the ground stations. Another station which, due to its momentarily favorable characteristics, for example a favorable signal to noise ratio, seems suitable, then becomes the reference station.
Systems of that type are still under investigation and have been published for example in the conference digest of "International Conference on Digital Satellite communications" Nov. 69 London and "L' Espace et la Communication" Paris 1971 Section 22.
In order to accomplish this change from one reference station to another information must be exchanged between the stations over a signal channel. However, the signal channel required for this purpose might be that information channel which is malfunctioning so that lack of synchronization cannot be prevented. Particularly when high transmitting and receiving frequency ranges are employed, it may happen that local atmospheric interference adversely influences reception of the code sequence SRUW transmitted by a certain reference station.